The present invention relates to a magnetic sensor, and more particularly to a magnetic sensor with a signal processing circuit, which is usable as a sensor such as a proximity switch, current sensor, or encoder.
As a magnetic sensor with a signal processing circuit, a Hall IC is well known which employs a Hall element as its sensor. As a typical conventional Hall IC, a silicon (Si) monolithic Hall IC (referred to as xe2x80x9cSi Hall ICxe2x80x9d from now on) has a magnetic sensor section in the form of a Hall element made of silicon (Si), and a signal processing IC section for processing a signal detected by the magnetic sensor section.
This type of the magnetic sensor has a low sensitivity to magnetic field because the magnetic sensor section of the Si Hall IC consists of the Hall element made of Si with a small electron mobility. Therefore, to operate the Hall IC as a magnetic sensor, large magnetic field must be applied to it. In other words, the Hall IC has a problem of a low sensitivity to magnetic field.
In addition, it is known that Si generates some voltage when mechanical stress is applied from outside.
Thus, the Si Hall IC has another problem of varying its sensitivity to magnetic field because of the voltage generated in the Hall element of the magnetic sensor section when external stress is applied.
Such problems must be considered when fabricating highly accurate, highly reliable proximity switches, current sensors or encoders by using the Si Hall IC.
Therefore, a magnetic sensor with a signal processing circuit is desired which can achieve accurate detection of a magnet position or magnetic field strength with a high sensitivity independent of external stress and with stable characteristics. Such a magnetic sensor has not yet been realized because of great difficulty.
On the other hand, various methods are studied to achieve highly accurate detection of the magnet position or magnetic field strength. For example, sensors, which use a Hall element as their magnetic sensor and include a signal processing circuit composed of discrete components such as an operational amplifier or resistors, are applied to the proximity switches, current sensors or encoders.
In these methods, however, users are required to have special technical expertise to understand the characteristics of the sensor, to implement optimum circuit design, and to acquire discrete components and assemble them. In addition, it is unavoidable that their cost and size increase because the sensors are implemented by mounting on a circuit board the magnetic sensor element and the signal processing circuit consisting of the discrete components. The cost and size increase presents a critical problem in the field of sensors that requires low cost and small size.
For example, in a conventional technique as shown in FIG. 12 of Japanese Patent Application Laid-open No. 38920/1990, the signal processing circuit comprises magnetoresistive elements 60 and 70 constituting a discrete magnetic sensor, resistors 6, 7 and 7xe2x80x2 and an operational amplifier 51. The feedback resistance is composed of the combined resistance of the resistors 6, 7 and 7xe2x80x2 with different temperature coefficients to form a feedback loop from the output terminal of the operational amplifier 51 to its inverting input terminal, thereby implementing magnetic characteristics with a desired temperature characteristic. This circuit configuration, however, has the foregoing problem of increasing the cost and size. Besides, the circuit configuration has another problem of decreased yield when obtaining an intended output because of the variations of the output voltage due to the variations in the midpoint potential of the magnetoresistive elements 60 and 70. Furthermore, since the midpoint potential usually drifts in accordance with temperature, the drift appears in the output voltage of the circuit, and has an adverse effect on the temperature characteristic of the output signal of the sensor. Thus, it is difficult for conventional circuits to obtain the desired temperature characteristic.
In addition, although the configuration of FIG. 12 can freely use the discrete resistors 6, 7 and 7xe2x80x2 with different temperature coefficients to implement the magnetic characteristics with the desired temperature characteristic, it discloses nothing about the implementation of a circuit like the Si monolithic IC.
Still another problem arises in that a common conventional Si Hall IC as shown in FIG. 14, which includes a signal processing circuit section 20a and a magnetic sensor section 30a that are electrically isolated from a substrate 21a only through the PN junction, for example, cannot perform stable operation in an ambient temperature above 125xc2x0 C., and cannot operate at all beyond 150xc2x0 C.
On the other hand, a technique is known which improves the temperature characteristics by reflecting the temperature dependence of the output resistance of a Hall element to a threshold voltage by employing the output resistance of the Hall element as the input resistance of a Schmitt trigger circuit. Specifically, in a circuit configuration as shown in FIG. 12, the threshold voltage Vth of the Schmitt trigger circuit is expressed as Vth=(Vdoxe2x88x92V1)xc2x7Rho/RF, where V1 is the potential at the inverting input terminal of the operational amplifier 51; RF is the feedback resistance; Vdo is the output potential of the amplified output signal 18 of the operational amplifier 51; and Rho is half the output resistance of the Hall element 4 (Japanese Patent Application Laid-open No. 226982/1986).
Here, the potential V1 causes a problem. Since the potential V1, which is the output potential of the Hall element 4, is about half the product of the input resistance Rhi of the Hall element 4 and the Hall element driving current Ic, the variations in Rhi causes the variations in V1. This in turn causes the variations in Vth, which makes it impossible to establish the threshold voltage exactly at a value designed. This results in the Hall IC with magnetic characteristics different from those designed, thereby reducing the yield.
The potential at the output terminal of the Hall element 4, which equals about half the input voltage to the Hall element, is referred to as a midpoint potential of the Hall element. The value has certain distribution due to the production variations of the Hall elements. The variations in the midpoint potential also cause the variations of V1, resulting in the reduction in the yield.
The inventors of the present invention intensively conducted the research to implement practical magnetic sensors capable of solving the foregoing problems of the magnetic sensor.
We aim to fabricate a highly sensitive, stable operation magnetic sensor with a signal processing circuit by forming the magnetic sensor with a structure of isolating the magnetic sensor section from the signal processing circuit consisting of a Si IC.
To implement a highly sensitive magnetic sensor with a signal processing circuit with stable operation characteristics, we study a magnetic sensor with a signal processing circuit that combines the signal processing circuit with a highly sensitive magnetic sensor composed of a compound semiconductor thin film or a magnetic thin film, which has a higher sensitivity in the magnetic field than the Si Hall element and can provide a stable magnetic sensor output independently of the mechanical external stress.
As a result, the inventors of the present invention invented a hybrid Hall IC which employed the compound semiconductor as the sensor, and combined it with a Si monolithic IC to be packed in a single package.
The present invention can implement a versatile, inexpensive, small size, high performance magnetic sensor with a signal processing circuit that does not require users to have any technical expertise such as special circuit technique, thereby making it possible to achieve detection of a magnet position or magnetic field strength at high accuracy.
In contrast with this, the conventional techniques cannot avoid the reduction in the yield of the Hall ICs because of the variations involved in producing the Hall elements or ICs. In addition, it cannot solve a problem of an increase in cost for improving the accuracy of circuit components of the ICs.
Furthermore, there is another problem in that as the temperature rises, the resistance increases of the compound semiconductor thin film or magnetic thin film constituting the magnetic sensor, and the output of the magnetic sensor reduces. Therefore, the magnetic sensor, when combined with the signal processing circuit without any change, has a problem of reducing the output of the magnetic sensor with a signal processing circuit as the temperature rises, that is, a problem of large dependence on temperature. This causes a critical problem in implementing highly accurate, practical detection because a magnet, a common object to be detected by the magnetic sensor with a signal processing circuit, has an inclination to reduce its magnetic flux density as the temperature rises.
The inventors of the present invention conducted researches to solve the problems.
The present invention is implemented to solve the problems, that is, to provide a magnetic sensor with a signal processing circuit without being affected from the magnetic sensor side. In other words, an object of the present invention is to prevent the reduction in the yield of the Hall IC due to the variations involved in producing the Hall elements or the variations in ICs, and to make it possible to reduce the number of components in the IC circuit and the demand for the accuracy, thereby achieving the improvement in the yield and reducing the cost.
Another object of the present invention is to implement a high performance magnetic sensor with a signal processing circuit having little dependence on temperature over a wide temperature range by correcting the temperature coefficients of the resistors and sensitivity of the magnetic sensor with a simple structure. Still another object of the present invention is to implement a high performance magnetic sensor with a signal processing circuit that can reduce the dependence of the sensor output on the temperature even if the magnetic field to be detected has large dependence on temperature as in the case of detecting the magnetic field of a permanent magnet.
In the first aspect of the present invention, there is provided a magnetic sensor with a signal processing circuit comprising:
a magnetic sensor section composed of one of a compound semiconductor thin film and a magnetic thin film; and
a signal processing circuit for amplifying a magnetic signal the magnetic sensor section detects as an electrical output,
wherein the signal processing circuit includes an operational amplifier and a constant current circuit for carrying out feedback.
Here, the constant current circuit may feed a different current value corresponding to an output of the operational amplifier back to an non-inverting input terminal of the operational amplifier.
The constant current circuit may include a plurality of resistors with at least two different temperature coefficients, and the current the constant current circuit outputs may have a temperature coefficient which is inversely proportional to a temperature coefficient of a combined resistance of the plurality of the resistors.
The combined resistance of the plurality of resistors may have a temperature coefficient that corrects a temperature coefficient of an internal resistance of the magnetic sensor section and a temperature coefficient of sensitivity of the magnetic sensor section.
The plurality of resistors may have temperature coefficients that correct not only the temperature coefficient of the internal resistance of the magnetic sensor section and the temperature coefficient of the sensitivity of the magnetic sensor section, but also a temperature coefficient of an object to be detected by the magnetic sensor section.
The signal processing circuit may be a monolithic IC.
The signal processing circuit may be formed on one of an insulated substrate and an insulating layer formed on a semiconductor substrate.
According to the second aspect of the present invention, a magnetic sensor with a signal processing circuit is provided that comprises:
a magnetic sensor section composed of one of a compound semiconductor thin film and a magnetic thin film; and
a signal processing circuit for amplifying a magnetic signal the magnetic sensor section detects as an electrical output,
wherein the signal processing circuit includes an operational amplifier and a constant current circuit for carrying out feedback, and is a monolithic IC.
The constant current circuit may feed a different current value corresponding to an output of the operational amplifier back to a non-inverting input terminal of the operational amplifier. Moreover, the constant current circuit includes a plurality of resistors with at least two different temperature coefficients. The current that the constant current circuit outputs has a temperature coefficient which is inversely proportional to a temperature coefficient of a combined resistance of the plurality of the resistors.
The combined resistance of the plurality of resistors has a temperature coefficient that corrects a temperature coefficient of an internal resistance of the magnetic sensor section and a temperature coefficient of sensitivity of the magnetic sensor section. The plurality of resistors have temperature coefficients that correct not only the temperature coefficient of the internal resistance of the magnetic sensor section and the temperature coefficient of the sensitivity of the magnetic sensor section, but also a temperature coefficient of an object to be detected by the magnetic sensor section.
According to the third aspect of the present invention, a magnetic sensor with a signal processing circuit comprising:
a magnetic sensor section composed of one of a compound semiconductor thin film and a magnetic thin film; and
a signal processing circuit for amplifying a magnetic signal the magnetic sensor section detects as an electrical output,
wherein the signal processing circuit includes an operational amplifier and a constant current circuit for carrying out feedback, and is formed on one of an insulated substrate and an insulating layer formed on a semiconductor substrate.
The constant current circuit may feed a different current value corresponding to an output of the operational amplifier back to a non-inverting input terminal of the operational amplifier. Moreover, the constant current circuit includes a plurality of resistors with at least two different temperature coefficients. The current that the constant current circuit outputs has a temperature coefficient which is inversely proportional to a temperature coefficient of a combined resistance of the plurality of the resistors.
The combined resistance of the plurality of resistors has a temperature coefficient that corrects a temperature coefficient of an internal resistance of the magnetic sensor section and a temperature coefficient of sensitivity of the magnetic sensor section. The plurality of resistors have temperature coefficients that correct not only the temperature coefficient of the internal resistance of the magnetic sensor section and the temperature coefficient of the sensitivity of the magnetic sensor section, but also a temperature coefficient of an object to be detected by the magnetic sensor section.
According to the fourth aspect of the present invention, a magnetic sensor with a signal processing circuit comprising:
a magnetic sensor section composed of one of a compound semiconductor thin film and a magnetic thin film; and
a signal processing circuit for amplifying a magnetic signal the magnetic sensor section detects as an electrical output,
wherein the signal processing circuit includes an operational amplifier and a constant current circuit for carrying out feedback, and is a monolithic IC, and is formed on one of an insulated substrate and an insulating layer formed on a semiconductor substrate.
In the fifth aspect of the present invention, there is provided a magnetic sensor with a signal processing circuit comprising:
a magnetic sensor section composed of one of a compound semiconductor thin film and a magnetic thin film; and
a signal processing circuit for amplifying a magnetic signal the magnetic sensor section detects as an electrical output,
wherein the signal processing circuit includes a plurality of feedback resistors with at least two different temperature coefficients, and the plurality of resistors feed an output of an operational amplifier back to its non-inverting input terminal.
Here, the magnetic sensor with a signal processing circuit in accordance with the present invention has the magnetic sensor section consisting of a compound semiconductor thin film, which can be any type of magnetic sensor that utilizes the Hall effect, magnetoresistance effect, or magnetic thin film based magnetoresistance effect. It is particularly preferable to utilize Hall elements or magnetoresistive elements which are composed of InAs (indium arsenic), GaAs (gallium arsenic), InGaAs (indium gallium arsenic), InSb (indium antimony), InGaSb (indium gallium antimony), etc., or magnetic thin film magnetoresistive elements composed of NiFe (nickel iron), NiCo (nickel cobalt), etc., or the magnetic sensors combining them.
Here, the compound semiconductor thin film refers to a thin film formed on a substrate by a common process technique of the semiconductor such as CVD (chemical vapor deposition), MBE (molecular beam epitaxy), vacuum evaporation, or sputtering, or to a thin film formed by shaving a semiconductor ingot, or to an active layer formed on the surface of a semiconductor substrate by ion implantation or diffusion.
The signal processing circuit of the magnetic sensor with a signal processing circuit in accordance with the present invention can be a common circuit produced with micro-structure. A circuit integrated on a Si substrate is preferable regardless of whether the circuit components have the MOS structure, bipolar structure, or hybrid structure thereof. Furthermore, as long as having the signal processing function, a circuit integrated on a GaAs substrate is also preferable. Moreover, a micro-structure circuit with a small size formed on a ceramic substrate is preferable, as well.
The foregoing plurality of resistors can have temperature coefficients that can correct not only the temperature coefficient of the internal resistance of the magnetic sensor section and the temperature coefficient of the sensitivity, but also the temperature coefficient of the object to be detected by the magnetic sensor section.
The signal processing circuit of the magnetic sensor of the signal processing circuit can be a circuit fabricated in micro-structure. For example, it can have such a structure that the circuit is formed on an insulated substrate like a circuit formed on a ceramic substrate. Alternatively, the signal processing circuit can be a circuit integrated on an insulating layer or high-resistance layer formed on a Si substrate. It can also be structured integrally with the semiconductor or ferromagnetic sensor formed on the surface of an IC.
The insulated substrate, insulating layer, or high-resistance layer refers to a substrate or a layer with a resistivity of equal to or more than 10 raised to the fifth to seventh power xcexa9xc2x7 cm excluding the PN junction insulation structure, such as a substrate or layer made of ceramic, silicon oxide or alumina.